Capsules, materials for use therein and electrophoretic media and displays containing such capsules

ABSTRACT

An encapsulation material, intended for use in encapsulated electrophoretic displays, comprises the coacervation product of a polyanionic polymer having a vinyl main chain and a plurality of anionic groups bonded to the main chain, with a cationic or zwitterionic water-soluble polymer capable of forming an immiscible second phase on contact with the polyanionic polymer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending application Ser. No.11/162,467, filed Sep. 12, 2005 (Publication No. 2006/0007528), which inturn is a divisional of application Ser. No. 10/249,966, filed May 22,2003 (Publication No. 2004/0012839, now U.S. Pat. No. 6,958,848), whichclaims priority from (a) Application Ser. No. 60/319,265, filed May 23,2002; (b) Application Ser. No. 60/319,342, filed Jun. 24, 2002; and (c)Application Ser. No. 60/319,343, filed Jun. 24, 2002.

This application is also related to (d) application Ser. No. 10/063,803,filed May 15, 2002 (Publication No. 2002/0185,378, now U.S. Pat. No.6,822,782); (e) copending application Ser. No. 10/063,236, filed Apr. 2,2002 (Publication No. 2002/0180687); and (f) application Ser. No.10/063,655, filed May 7, 2002 (Publication No. 2002/0171190, now U.S.Pat. No. 6,870,661). The entire contents of all these applications, andof all U.S. patents and published applications mentioned below, areherein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to capsules and materials for use therein. Thecapsules of the present invention are especially, but not exclusively,intended for use in electrophoretic displays. This invention alsorelates to binders for use in electrophoretic displays. This inventionalso relates to processes for forming electrophoretic media anddisplays, and to the media and displays so formed.

Electrophoretic displays have been the subject of intense research anddevelopment for a number of years. Such displays can have attributes ofgood brightness and contrast, wide viewing angles, state bistability,and low power consumption when compared with liquid crystal displays.(The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element.)Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; and 6,545,291; and U.S.Patent Applications Publication Nos. 2002/0019081; 2002/0021270;2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980;2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792;2002/0154382, 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378;2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855;2003/0034949; 2003/0038755; and 2003/0053189; and InternationalApplications Publication Nos. WO 99/67678; WO 00/05704; WO 00/20922; WO00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO00/67327; WO 01/07961; and WO 01/08241.

Known electrophoretic media, both encapsulated and unencapsulated, canbe divided into two main types, referred to hereinafter for convenienceas “single particle” and “dual particle” respectively. A single particlemedium has only a single type of electrophoretic particle suspending ina colored suspending medium, at least one optical characteristic ofwhich differs from that of the particles. (In referring to a single typeof particle, we do not imply that all particles of the type areabsolutely identical. For example, provided that all particles of thetype possess substantially the same optical characteristic and a chargeof the same polarity, considerable variation in parameters such asparticle size and electrophoretic mobility can be tolerated withoutaffecting the utility of the medium.) The optical characteristic istypically color visible to the human eye, but may, alternatively or inaddition, be any one of more of reflectivity, retroreflectivity,luminescence, fluorescence, phosphorescence, or color in the broadersense of meaning a difference in absorption or reflectance atnon-visible wavelengths. When such a medium is placed between a pair ofelectrodes, at least one of which is transparent, depending upon therelative potentials of the two electrodes, the medium can display theoptical characteristic of the particles (when the particles are adjacentthe electrode closer to the observer, hereinafter called the “front”electrode) or the optical characteristic of the suspending medium (whenthe particles are adjacent the electrode remote from the observer,hereinafter called the “rear” electrode, so that the particles arehidden by the colored suspending medium).

A dual particle medium has two different types of particles differing inat least one optical characteristic and a suspending fluid which may beuncolored or colored, but which is typically uncolored. The two types ofparticles differ in electrophoretic mobility; this difference inmobility may be in polarity (this type may hereinafter be referred to asan “opposite charge dual particle” medium) and/or magnitude. When such adual particle medium is placed between the aforementioned pair ofelectrodes, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of eitherset of particles, although the exact manner in which this is achieveddiffers depending upon whether the difference in mobility is in polarityor only in magnitude. For ease of illustration, consider anelectrophoretic medium in which one type of particles is black and theother type white. If, the two types of particles differ in polarity (if,for example, the black particles are positively charged and the whiteparticles negatively charged), the particles will be attracted to thetwo different electrodes, so that if, for example, the front electrodeis negative relative to the rear electrode, the black particles will beattracted to the front electrode and the white particles to the rearelectrode, so that the medium will appear black to the observer.Conversely, if the front electrode is positive relative to the rearelectrode, the white particles will be attracted to the front electrodeand the black particles to the rear electrode, so that the medium willappear white to the observer.

If the two types of particles have charges of the same polarity, butdiffer in electrophoretic mobility (this type of medium may hereinafterto referred to as a “same polarity dual particle” medium), both types ofparticles will be attracted to the same electrode, but one type willreach the electrode before the other, so that the type facing theobserver differs depending upon the electrode to which the particles areattracted. For example suppose the previous illustration is modified sothat both the black and white particles are positively charged, but theblack particles have the higher electrophoretic mobility. If now thefront electrode is negative relative to the rear electrode, both theblack and white particles will be attracted to the front electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the front electrodeand the medium will appear black to the observer. Conversely, if thefront electrode is positive relative to the rear electrode, both theblack and white particles will be attracted to the rear electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the rear electrode,leaving a layer of white particles remote from the rear electrode andfacing the observer, so that the medium will appear white to theobserver: note that this type of dual particle medium requires that thesuspending fluid to sufficiently transparent to allow the layer of whiteparticles remote from the rear electrode to be readily visible to theobserver. Typically, the suspending fluid in a dual particle display isnot colored at all, but some color may be incorporated for the purposeof correcting any undesirable tint in the white particles seentherethrough.

Both single and dual particle electrophoretic displays may be capable ofintermediate gray states having optical characteristics intermediate thetwo extreme optical states already described. It is shown in theaforementioned application Ser. No. 10/063,236 that some electrophoreticdisplays are stable not only in their extreme optical states but also intheir intermediate gray states. This type of display is properly called“multi-stable” rather than bistable, but the latter term may be usedherein for convenience.

Some of the aforementioned patents and published applications discloseencapsulated electrophoretic media having three or more different typesof particles within each capsule. For purposes of the presentapplication, such multi-particle media are regarded as sub-species ofdual particle media.

Also, many of the aforementioned patents and applications recognize thatthe walls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, the aforementioned2002/0131147. Accordingly, for purposes of the present application, suchpolymer-dispersed electrophoretic media are regarded as sub-species ofencapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively. Present dayelectrophoretic displays exhibit paper-like reflective optics, extremelylow power consumption due to retained image capability, and mechanicalconformability and flexibility.

Although electrophoretic displays are often opaque (since the particlessubstantially block transmission of visible light through the display)and operate in a reflective mode, electrophoretic displays can be madeto operate in a so-called “shutter mode” in which the particles arearranged to move laterally within the display so that the display hasone display state which is substantially opaque and one which islight-transmissive. See, for example, the aforementioned U.S. Pat. Nos.6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361;6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, whichare similar to electrophoretic displays but rely upon variations inelectric field strength, can operate in a similar mode; see U.S. Pat.No. 4,418,346. Other types of electro-optic displays may also be capableof operating in shutter mode.

However, the environments in which an encapsulated electrophoreticdisplay can be used is determined, at least in part, by thecharacteristics of the materials used to form the walls of themicrocapsules present in the display, and prior art microcapsules dohave some limitations in this regard. The aforementioned ApplicationsSerial Nos. 10/063,803, 10/063,236 and 10/063,655 describe formation ofmicrocapsule walls by coacervation of gelatin and acacia, followed bycross-linking with glutaraldehyde. The resulting microcapsules have anoperating temperature range of about +10 to +60° C., may burst attemperatures near the upper end of this range, and are sufficientlysensitive to humidity that the optical performance of theelectrophoretic displays is adversely affected at combinations of hightemperature and high humidity such as might be encountered in a tropicalrain forest environment. Although it might at first appear that suchmicrocapsules could be made to operate at higher temperatures simply byincreasing the thickness of the microcapsule wall, increased wallthickness may result in poorly packed films of the microcapsules and/orless deformable microcapsules, and both these effects aredisadvantageous in electrophoretic media, as discussed in more detailbelow. Accordingly, there is a need for improved microcapsule wallmaterials to expand the operating limits of such electrophoreticdisplays. In particular, there is a need for improved microcapsule wallmaterials which will permit electrophoretic displays to operatesatisfactorily at extreme temperature and humidity, and thus meet thehigh performance needs of military and commercial mobile deviceapplications.

However, the search for new microcapsule wall materials useful inelectrophoretic and similar displays is complicated by the need for thematerial to meet the numerous requirements necessary in practicalproduction of such displays. Among the requirements are:

(a) The encapsulation procedure must be reproducible and manufacturable,involve inexpensive raw materials, and yield capsules that are totallyimpermeable to their contents;

(b) The microcapsules must be amenable to coating. While the propertiesof a microencapsulated dispersion that allow facile, uniform coating arenot entirely understood, one property that is important is flexibilityof the capsule wall. If the wall is too rigid, the coating suspensionshows severe shear-thickening rheological behavior, and is eitherimpossible to coat because of hopper jamming or yields very non-uniformcoatings. Flexibility of the capsule wall also allows closer packing inthe coating, and thus yields displays with improved optical properties;

(c) The capsule wall must have mechanical, optical, and electricalproperties that allow the construction of a durable display with rapidresponse at low driving voltages. In particular, the shell must betolerant to mechanical deformation (this is especially important forflexible display applications) and must not be appreciably colored oropaque. Also, the electrical resistance of the shell wall material mustbe high; a capsule wall with poor electrical properties can short outthe display; and

(d) The capsule wall must maintain its properties over a wide range ofoperating conditions. The response of the capsule to changes in humidityis especially problematical, since it has been found to be difficult toachieve simultaneously all of the characteristics listed above with acapsule wall composition whose electrical conductivity is sufficientlyinsensitive to high ambient humidity. Improvements in the environmentalsensitivity of the capsule wall represent a major contribution to therobustness of the display.

Furthermore, most microencapsulation techniques known in the literatureare intended for controlled release of the capsule contents, so that themicrocapsule is intended to break or become selectively permeable inuse. Hence, materials developed for other types of microcapsules may notbe useful for microcapsules to be used in electrophoretic displays,where capsules must perform in ways that are highly atypical, in thatthey are intended to provide permanent encapsulation of their contents.

As described in the aforementioned MIT and E Ink patents and publishedapplications, a microencapsulated electrophoretic medium is typicallyformed by mixing microcapsules with a solution containing a polymerbinder, laying down a layer of the resultant microcapsule/bindersolution mixture on a substrate, and drying the layer to produce anelectrophoretic medium in which the microcapsules are embedded in alayer of the polymer binder. The substrate bearing the electrophoreticmedium is then typically laminated, using a lamination adhesive to abackplane arranged to apply drive voltages to the medium. The binderimproves the mechanical integrity of the layer of microcapsules, and mayimprove the adhesion of the microcapsules to the substrate on which theyare deposited. It has now been found that the sensitivity ofelectrophoretic media to humidity can be significantly reduced bymodifying the binder and/or the lamination adhesive rather than thematerial used to form the microcapsules walls.

It is also desirable to reduce the operating voltage ofmicroencapsulated electrophoretic displays. Considerable progress hasalready been made in this regard; some of the early displays describedin the aforementioned E Ink and MIT patents and applications needed tobe operated at 90 V, whereas similar displays can now operate at only 15V. However, further reduction in operating voltage is still desirable,because reducing the operating voltage reduces the energy consumption ofthe display, an important factor in displays intended for portabledevices. Also, when it is desired to drive a display using dry cells orsimilar small batteries, which only generate (say) 1.5 to 6 V DC, evenoperating a display at 15 V requires the provision of special circuitryto step up the DC voltage produced by the battery to that required bythe display. If the operating voltage of the display could be reduced tothat produced by the battery, this circuitry could be eliminated and thecost of the display reduced.

As already mentioned, in an encapsulated electrophoretic display themicrocapsules which form the electrophoretic medium are typicallyenclosed in a binder. The microcapsules/binder layer is typicallysandwiched between two electrodes (or, in some cases, between anelectrode and a non-electrode support member, a movable electrode beingmoved over the support member to address the display), it normally alsobeing necessary to include a layer of a lamination adhesive between theelectrodes to ensure the mechanical integrity of the display. Apotential difference is applied between the electrodes to address thedisplay. Since the switching of an electrophoretic medium is dependentupon the electric field across the medium, the operating voltagerequired by a display can be reduced by reducing the thickness of themicrocapsules/binder layer, since a thinner layer enables the sameelectric field, and hence the same electro-optic response of themicrocapsules, to be produced at a lower operating voltage.

A further problem with some electrophoretic displays is the phenomenonknown as “self-erasing”; see, for example, Ota, I., et al.,“Developments in Electrophoretic Displays”, Proceedings of the SID, 18,243 (1977), where self-erasing was reported in an unencapsulatedelectrophoretic display. When the voltage applied across certainelectrophoretic displays is switched off, the electrophoretic medium mayreverse its optical state, and in some cases a reverse voltage, whichmay be larger than the operating voltage, can be observed to occuracross the electrodes. It appears (although this invention is in no waylimited by this belief) that the self-erasing phenomenon is due to amismatch in electrical properties between various components of thedisplay; in particular, in the case of an encapsulated electrophoreticdisplay, it appears that the phenomenon is due to a mismatch inelectrical properties between the internal phase of the microcapsulesand the polymer layer, namely the microcapsule walls, which is inelectrical series with this internal phase. Obviously, self-erasing ishighly undesirable in that it reverses (or otherwise distorts, in thecase of a grayscale display) the desired optical state of the display.

Another problem sometimes encountered with encapsulated electrophoreticdisplays is that, after the display has been operating for an extendedperiod, the electrophoretic particles may tend to stick to the interiorsurfaces of the microcapsules, thus ceasing to move when an electricfield is applied to the display and the optical contrast between theoptical states of the display.

The present invention seeks to provides capsule wall materials,capsules, and encapsulated electrophoretic media and displays, in whichthe aforementioned problems are reduced or eliminated, and which thusexpand the operating range of electrophoretic displays. The capsule wallmaterials provided by the present invention may be useful forencapsulation of materials other than electrophoretic media, for examplepharmaceuticals.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides an encapsulationmaterial comprising the coacervation product of a polyanionic polymerhaving a vinyl main chain and a plurality of anionic groups bonded tothe main chain, with a cationic or zwitterionic water-soluble polymercapable of forming an immiscible second phase on contact with thepolyanionic polymer.

This aspect of the invention may hereinafter for convenience be referredto as the “polyanionic-based encapsulation material” of the invention.

In this encapsulation material, the cationic water-soluble polymer maycomprises a protein, preferably gelatin. The encapsulation material maybe cross-linked with an aldehyde, for example glutaraldehyde. Theanionic groups may be, for example, any one or more of sulfate,sulfonate, phosphate, carboxylic acid and carboxylate groups.

A preferred group of polymers for use in the polyanionic-basedencapsulation material of the invention are those of the formula:

where x and y are the mole fractions of the two monomer residues in thepolymer and total 1, one or more of R₁ to R₈ is an anionic group, andthose of R₁ to R₈ which are not anionic groups are hydrogen, saturatedhydrocarbon groups, groups of the formula —OR₉ or —COOR₁₀ (wherein R₉and R₁₀ are hydrocarbon groups), aryl, substituted aryl or halocarbongroups. Among this group of polyanionic polymers, preferred sub-groupsare those containing a group of formula —OR₉ derived from a vinyl etheror vinyl carboxylate ester, those containing a group of formula —COOR₁₀derived from an acrylate or methacrylate ester, those in which at leastone of the groups R₁ to R₈ is a styrene sulfonic acid or styrene sodiumsulfonate group, and those in which at least one of the groups R₁ to R₈is a vinyl chloride or vinylidene chloride grouping. The polyanionicpolymer may, for example, comprise any one or more of poly(acrylicacid); poly(methacrylic acid); copolymers of poly(acrylic acid) and/orpoly(methacrylic acid) with esters of the same acids; styrene sulfonatecopolymers with styrene; methyl vinyl ether or vinyl acetate copolymerswith (meth)acrylic acid; copolymers of alkyl-substituted olefins, methylvinyl ether and vinyl carboxylate with maleic acid, maleic esters, andmaleic half ester, half acids.

The present invention also provides a capsule having an internal phaseand a wall formed from a polyanionic-based encapsulation material of theinvention. In this capsule, the internal phase may comprise a liquid,preferably a liquid hydrocarbon, alone or in combination with ahalocarbon. The internal phase may also comprise a plurality of chargedparticles capable of moving through the liquid on application of anelectric field to the capsule.

The present invention also provides a process for encapsulating ainternal phase, which process comprises contacting the internal phasewith a polyanionic polymer having a vinyl main chain and a plurality ofanionic groups bonded to the main chain and with a cationic orzwitterionic water-soluble polymer capable of forming an immisciblesecond phase on contact with the polyanionic polymer. the contact beingeffected under conditions effective to cause formation around theinternal phase of a capsule wall comprising a coacervation product ofthe two polymers. Desirably, this process also comprises cross-linkingthe capsule wall with an aldehyde, for example, glutaraldehyde.

The present invention also provides an electrophoretic medium comprisinga plurality of capsules, each capsule comprising a plurality of chargedparticles suspended in a suspending fluid and capable of moving throughthe fluid on application of an electric field to the capsule, eachcapsule further comprising a wall surrounding the charged particles andthe suspending fluid, the wall comprising a polyanionic-basedencapsulation material of the present invention.

The present invention also provides an electrophoretic displaycomprising a layer of an electrophoretic medium of the invention asdefined above, and at least one electrode disposed adjacent theelectrophoretic medium and arranged to apply an electric field thereto.

In another aspect, this invention provides an electrophoretic mediumcomprising a plurality of capsules in a polymeric binder. Each capsulecomprises a capsule wall and an internal phase encapsulated by thecapsule wall, the internal phase comprising a suspending fluid and aplurality of electrically charged particles suspended in the suspendingfluid and capable of moving therethrough upon application of an electricfield to the capsule. The polymeric binder is a water soluble polymer,preferably gelatin. This aspect of the invention may hereinafter calledthe “water soluble binder” electrophoretic medium.

In such a water soluble binder electrophoretic medium, the water solublepolymer may comprise gelatin, preferably in the form of coacervate ofgelatin and acacia.

In another aspect, the invention provides a capsule comprising a capsulewall and an internal phase encapsulated by the capsule wall, theinternal phase comprising a suspending fluid and a plurality ofelectrically charged particles suspended in the suspending fluid andcapable of moving therethrough upon application of an electric field tothe capsule. The capsule wall is formed of a material which is swellableby the suspending fluid. This aspect of the invention may hereinaftercalled the “swellable wall” capsule.

In such a swellable wall capsule, the internal phase may comprise ahydrocarbon and the capsule wall material comprise any one or more of asilicone, a polymer derived from a vinylic monomer, and a polyurethane.For example, the capsule wall material may comprise any one or more ofpoly(dimethyl siloxane), poly(trifluorobutyl methyl siloxane),poly(vinyl chloride), poly(butadiene), a polyacrylate, and apolymethacrylate. Also, in a swellable wall capsule, for reasonsdescribed in detail below, the internal surface of the capsule wall maybear at least one polymer chain which is solvated by the suspendingfluid. In one form of such a capsule, the internal phase comprises ahydrocarbon and the polymer chain has a main chain and a plurality ofside chains extending from the main chain, each of the side chainscomprising at least about four carbon atoms.

This invention extends to an electrophoretic medium comprising aplurality of swellable wall capsules and a binder surrounding thecapsules. Desirably, the binder is substantially not swellable by thesuspending fluid. If, as is commonly the case, the electrophoreticmedium also comprises a layer of an adhesive, this adhesive is desirablynot substantially swellable by the suspending fluid.

In another aspect, the invention provides an electrophoretic mediumcomprising a plurality of capsules in a polymeric binder. Each capsulecomprises a capsule wall and an internal phase encapsulated by thecapsule wall, the internal phase comprising a suspending fluid and aplurality of electrically charged particles suspended in the suspendingfluid and capable of moving therethrough upon application of an electricfield to the capsule. The electrophoretic medium may optionally comprisea layer of a lamination adhesive in contact with the binder. In thisaspect of the present invention, at least one of the polymeric binderand lamination adhesive is formed from a blend of two (or more)materials, preferably polymers, the blend having lower changes in volumeresistivity with humidity than either component separately. This aspectof the invention may hereinafter called the “mixed binder/adhesive”electrophoretic medium.

In such a mixed binder/adhesive electrophoretic medium, the blend maycomprise a plurality of polyurethanes.

Finally, in another aspect, this invention provides a process forforming a capsule.

This process comprises:

providing a liquid internal phase comprising a fluid and a firstprepolymer dispersed therein and having a plurality of first reactivegroups;

providing a dispersion medium substantially immiscible with the internalphase and comprising a dispersing liquid and a second prepolymerdispersed therein and having a plurality of second reactive groups, eachof the second reactive groups being capable of reacting with at leastone of the first reactive groups; and

dispersing the internal phase as a plurality of discrete droplets in thedispersion medium, thereby causing the first and second reactive groupsto react together to form a polymer shell surrounding the droplets, andthereby forming capsules comprising the fluid.

This process may hereinafter be called the “two prepolymer” process ofthe invention. In one form of such a two prepolymer process, theinternal phase further comprises a plurality of electrically chargedparticles capable of moving through the internal phase upon applicationof an electric field thereto.

In the two prepolymer process, the dispersion medium may be an aqueousmedium and the internal phase an organic medium. The two reactive groupsmay comprise an acid anhydride grouping and an amine group, which reacttogether to form amide linkages. An example of two prepolymerscontaining such reactive groups are poly(isoprene-graft-maleicanhydride) and polyethyleneimine.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the present invention will now be described,though by way of illustration only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a photomicrograph of a layer of microcapsules and bindersolution being used to prepare a water soluble binder electrophoreticmedium of the present invention;

FIG. 2 is a photomicrograph, similar to that of FIG. 1, showing thefinal electrophoretic medium prepared from the microcapsules and binderof FIG. 1;

FIG. 3 is a photomicrograph, similar to that of FIG. 2, but showing acontrol medium which does not use contain a water soluble binder;

FIGS. 4 and 5 are graphs showing the variations of volume resistivitywith relative humidity for certain polyurethanes and blends thereof, asdetermined in Examples 5 and 6 below; and

FIGS. 6 and 7 are photomicrographs of capsules produced by a twoprepolymer process of the present invention, FIG. 6 showing the capsulesdispersed in water and

FIG. 7 showing the capsules in the form of a dried film.

DETAILED DESCRIPTION

As already indicated, the present invention provides a number ofimprovements in capsules, materials for use therein, processes for theirpreparation, and electrophoretic media and displays using them. Thevarious aspects of the invention will now be described sequentially, butit should be recognized that a single electrophoretic medium or displaymay make use of more than one aspect of the invention. For example, anelectrophoretic display might use a polyanionic-based encapsulationmaterial of the present invention in combination with a water solublebinder. The following discussion will conclude with a general discussionof various considerations regarding capsules for use in electrophoreticdisplays.

Polyanionic-Based Encapsulation Material

As already mentioned, this invention provides an polyanionic-basedencapsulation material comprising the coacervation product of apolyanionic polymer having a vinyl main chain and a plurality of anionicgroups bonded to the main chain and a cationic or zwitterionicwater-soluble polymer capable of forming an immiscible second phase oncontact with the polyanionic polymer. The encapsulation material ispreferably cross-linked with an aldehyde, for example glutaraldehyde.The polyanionic-based encapsulation materials of the present inventionhave been found to be less sensitive to changes in humidity than theprior art encapsulation materials formed from gelatin and acacia, asalready described. When used in electrophoretic displays, thepolyanionic-based encapsulation materials have surprising effects uponthe electro-optic properties of the display; in particular, it has beenfound that the electro-optic properties of the display are lesssensitive to humidity and moisture. Capsules prepared with preferredpolyanionic-based encapsulation materials of the present invention havebeen found to tolerate temperatures up to about 100° C. withoutbursting.

The anionic groups in the polyanionic-based encapsulation materials ofthe present invention may be, for example, sulfate (—OSO₃—), sulfonate(—SO₃—), phosphate (—OP(O)(OH)(O—) or —OP(O)(OH)(O—)₂), or carboxylicacid or carboxylate (—COOH or COO—) groups. The optimum proportion ofanionic groups for any specific application may readily be determinedempirically, but should be large enough that the polymer used to formthe encapsulation material is water soluble, at least at high pH's ofabout 10 or higher, when all of the anionic groups are ionized. As iswell known to those skilled in the part of polymer synthesis, theproportion of anionic groups in the polymer can readily be varied byvarying the proportions of the monomers used to form the polymer.

In the preferred polymers of Formula I above, when one of the groups R₁to R₈ is of the formula —OR₉, it may be a group derived from a vinylether or vinyl carboxylate ester, for example vinyl acetate. Similarly,when one of the groups R₁ to R₈ is of the formula —COOR₁₀, it may be agroup derived from an acrylate or methacrylate ester. When one of thegroups R_(1 to R) ₈ is an aryl sulfonate, it may be a styrene sulfonicacid or styrene sodium sulfonate group. Finally, when one of the groupsR₁ to R₈ is a halocarbon group, it may be a vinyl chloride or vinylidenechloride group.

Preferred polymers for use in the polyanionic-based encapsulationmaterials of the present invention include poly(acrylic acid);poly(methacrylic acid); copolymers of poly(acrylic acid) and/orpoly(methacrylic acid) with esters of the same acids (i.e., poly(acrylicacid)-co-(butyl acrylate)); styrene sulfonate copolymers with styrene;methyl vinyl ether or vinyl acetate copolymers with (meth)acrylic acid;copolymers of alkyl-substituted olefins, methyl vinyl ether and vinylcarboxylate (e.g., vinyl acetate) with maleic acid, maleic esters, andmaleic half ester, half acids. Examples of this last group of polymersinclude hydrolyzed poly(ethylene)-alt-(maleic anhydride), hydrolyzedpoly(isobutylene)-alt-(maleic anhydride), hydrolyzed poly(methyl vinylether)-alt-(maleic anhydride), and other similar polymers. Solvolysis ofthe same maleic anhydride copolymers with simple alcohols giveshalf-ester, half-acid copolymers with widely varyinghydrophobic-hydrophilic balance, which may be useful in the presentinvention. In all of these cases, the acid (anionic group) content ofthe polymer is sufficient to assure adequate water solubility, so thatthe coacervate phase can be made. However, the range of useful materialscan be extended by the inclusion of a certain amount (between 0 and 50%)of a water-soluble co-solvent, such as methanol, tetrahydrofuran,dimethyl sulfoxide, dimethylformamide, acetone, or other water-miscibleorganic material, which both enhances the aqueous solubility of thecoacervating anion, and which at the same time enhances the stability ofthe coacervate complex. Salts may be included in the microencapsulationmedium to moderate coacervate formation.

As already mentioned, the second polymer used to form thepolyanionic-based encapsulation material may be any cationic orzwitterionic water-soluble polymer capable of forming an immisciblesecond phase on contact with the polyanionic polymer. This secondpolymer may be, for example, a vinyl (addition) polymer comprisingcationic functional groups, or a cationic condensation polymer, such aspolyethylene imine, a cationic polyester, polyurethane, polyether or thelike. However, the preferred second polymers for use in the presentinvention are cationic proteins, the specific preferred material beinggelatin.

Apart from the use of a polyanionic-based encapsulation material,polyanionic-based electrophoretic media and displays of this inventioncan make use of any of the materials and production techniques describedin the aforementioned MIT and E Ink applications, to which the reader isreferred for additional details.

The following Examples are now given, though by way of illustrationonly, to show details of particular preferred materials and processesused in the polyanionic-based encapsulation materials, electrophoreticmedia and displays of the present invention.

EXAMPLE 1 Electrophoretic Display Using Poly(Acrylic Acid)/GelatinEncapsulation Material

This Example illustrates the preparation of a polyanionic-basedencapsulation material, electrophoretic medium and electrophoreticdisplay of the present invention, the encapsulation material beingformed by coacervation of poly(acrylic acid) and gelatin.

In a 4 L reactor equipped with a stirrer, gelatin (49.2 g) was dissolvedin deionized water (2622.4 g) at 45° C. Separately, poly(acrylic acid)(molecular weight 250,000, 2.7 g of a 35 weight percent solution inwater, available from Sigma-Aldrich) was dissolved in deionized water(655.6 g) and the resultant solution heated to 45° C. Also separately,an internal phase (1060 g), comprising white and black pigment particlessuspended in Isopar G (prepared substantially as described in theaforementioned application Ser. No. 10/063,236, Paragraphs[0069]-[0070]) was heated to 45° C. and then added, over a period ofapproximately 10 minutes, to the stirred gelatin solution. The additionwas conducted by introducing the internal phase through a droppingfunnel, the outlet of which was placed below the surface of the gelatinsolution. After the addition of the internal phase was complete, therate of stirring was increased to 580 rpm and the stirring continued for60 minutes at 45° C. in order to emulsify the internal phase intodroplets having an average diameter of about 40 μm.

The warm poly(acrylic acid) solution was then added over a period ofabout 1 minute. After the addition was complete, the pH of the mixturewas raised to 6.3 using 1 percent aqueous ammonium hydroxide, and thestirring was continued for a further 40 minutes. The temperature of themixture was then lowered to 10° C. over a period of two hours, withcontinued stirring, and 16.7 g of a 50% solution of glutaraldehyde wasadded. After this addition, the mixture was gradually warmed to 25° C.and stirred for a further 12 hours.

The liquid phase was then removed and the capsules in this liquid phasewashed by sedimentation and decantation, followed by redispersion indeionized water. The capsules were separated according to size by wetsieving to yield a distribution between 20 and 60 μm diameter, with amean diameter of about 40 μm. Such a distribution can be achieved bysieving the capsules for 90 seconds on a 38 μm sieve and then for 90seconds on a 25 μm sieve. The resulting capsule slurry was concentratedby centrifugation and decantation and then mixed with an aqueousurethane binder at a ratio of 1 part by weight binder to 8 parts byweight of wet capsules. This slurry was mixed with 0.3 weight percent ofhydroxypropylmethylcellulose (molecular weight 86,000), and 0.1 weightpercent Triton X-100 as slot-coating additives. The resultant mixturewas slot-coated on to a 175 μm thick indium-tin oxide coated polyesterfilm. The coated film was oven dried at 50° C. for 10 minutes to producean electrophoretic medium comprising a coating of electrophoreticcapsules approximately 20 μm thick, this coating comprising essentiallya single layer of capsules (see the aforementioned WO 00/20922).

The resultant coated film was then assembled in the following mannerinto a polyanionic-based electrophoretic display of the presentinvention. A polyurethane adhesive (three different adhesives were used,as described in Example 3 below), was coated on a polyethyleneterephthalate release sheet using a slot-die coater. The coated releasesheet was transferred to an oven at 65° C. and dried for 10 minutes.During coating, the flow rate through the slot, and the coating-headspeed were adjusted to provide a film of adhesive that measured 15 μmthick when dry. The coated and dried release sheet was then laminated tothe microcapsule-coated polyester film using a Western Magnum rolllaminator; the dried release sheet was laid on top of the microcapsulelayer and laminated in the nip of the laminator at 50 psig (0.46 mPa),with the upper roll at 300° F. (149° C.) and the lower roll at 275° F.(135° C.), at a linear speed of 0.7 ft/min (3.5 mm/sec). The resultinglaminate was then cooled, and a single-pixel display produced by cuttinga piece of appropriate size from the cooled laminate, removing therelease sheet, and laying the film, adhesive side down, on a rearelectrode and passing the resultant combination through the laminatorusing the same conditions as before.

EXAMPLE 2 (Control): Electrophoretic Display Using Gum Acacia/GelatinEncapsulation Material

This Example illustrates the preparation of a prior art encapsulationmaterial, electrophoretic medium and electrophoretic display of the typedescribed above in which the encapsulation material is formed bycoacervation of gum acacia and gelatin.

Encapsulation of the same internal phase as in Example 1 above wascarried out in substantially the same manner as in that Example, exceptthat gum acacia was used in place of poly(acrylic acid). Afteremulsification of the internal phase in gelatin, a solution of acacia(66.7 g, supplied by AEP Colloids, Inc.) in water (656 mL) was addedover a period of about 1 minute, and the pH of the mixture was loweredto approximately 4.9 using 10 percent aqueous acetic acid. Aconcentrated capsule slurry was prepared by washing and sedimentation asdescribed in Example 1, and the pH of the slurry adjusted to pH 8 with 1weight percent ammonium hydroxide solution. Capsules were concentratedby centrifugation and then mixed with an aqueous urethane binder at aratio of 1 part by weight binder to 8 parts by weight of capsules. Thecoating of the capsules and the production of the electrophoretic mediumand display were identical to those of Example 1.

EXAMPLE 3 Comparison of Electro-Optic Performance of Displays Under HighHumidity

This Example illustrates the improved electro-optic performance of theelectrophoretic display of the present invention prepared in Example 1above under high humidity conditions, as compared with the prior artdisplay prepared in Example 2 above.

The electro-optic performance of the displays prepared in Examples 1 and2 above was evaluated after equilibration of the displays for two weeksin humidity-controlled chambers at 25° C./30% relative humidity (RH) and25° C./70% RH. The displays were driven using 15 V switching pulses, andthe reflectivity of the pixels to white light in the white and blackstates was measured. During these measurements, the length of theswitching pulses was adjusted to achieve optical saturation of thedisplays; for the displays equilibrated at 30% RH, a pulse length of 300msec was used, whereas for those equilibrated at 70% RH, pulse lengthsup to 600 msec were employed, although optical saturation of thegelatin/acacia display was not always achieved even after this time,whereas the displays of the present invention achieved opticalsaturation using pulse lengths that showed little change between 30% and70% RH. The contrast ratio (CR), defined as the ratio of thereflectivity in the white state to that in the dark state, was used as ameasure of the electro-optic performance of the displays. The resultsare shown in the Table below.

As indicated in the Table, three different lamination adhesives wereused. Adhesive 1 consisted of 60% by weight Neorez R9630 blended with40% by weight Neorez R9330 (Neorez R9630 and Neorez R9330 arepolyurethane latex suspensions supplied by Neoresins, Inc.). Adhesive 2consisted of 50% by weight Neorez R9630 blended with 50% by weightNeorez R9330, while Adhesive 3 consisted of 40% by weight Neorez R9630blended with 60% by weight Neorez R9330. TABLE CR 30% RH, CR 70% RH, CR70% RH, Example Adhesive 300 ms pulse 300 ms pulse 600 ms pulse 1 1 9.43.4 5.8 2 (Control) 1 12.0 1.0 1.2 1 2 8.8 5.5 8.1 2 (Control) 2 10.51.0 1.7 1 3 9.4 4.2 6.7 2 (Control) 3 12.0 1.0 2.9

From the data in this Table, it will be seen that all the displaysperformed reasonably well at 30% RH, showing a contrast ratio of from8.8 to 12.0 (which is sufficient to give clear rendition of text andother black and white information) and a rapid response time of 300msec. When equilibrated at high (70%) RH, the Control displays showedessentially no switching, with contrast ratios of 1.0, whereas thedisplays of the present invention, using poly(acrylic acid) instead ofgum acacia, still showed good switching, with contrast ratios greaterthan 3.4. The contrast ratios for the displays of the invention could befurther improved by using a pulse length of 600 msec, whereas theControl displays still showed very poor electro-optic response at thisincreased pulse length. With an appropriate adhesive (Adhesive 2 in theTable), the response of the displays of the invention could be madealmost as good at high RH as at low RH, if the slower switching timewere accepted.

Water Soluble Binder Electrophoretic Media

As already mentioned, this invention provides an encapsulatedelectrophoretic medium in which the binder is a water-soluble (notsimply water-dispersible) polymer, preferably a water soluble proteinand desirably gelatin. In the MIT and E Ink patents and applicationsmentioned above, the binders shown in the working Examples are typicallypolyurethane latices. Although such latices are aqueous, thepolyurethanes therein are not water-soluble and are present in the latexas a discrete solid phase, which usually requires a substantial amountof surfactant to form a stable latex. In contrast, the materials used inthe water soluble binder aspect of the present invention are truly watersoluble, so that the binder solution used in a true solution.

It has been found that using a water soluble binder rather than a latexto form the electrophoretic medium allows the production of thinelectrophoretic media, and thus (for reasons previously discussed) loweroperating voltages, or alternatively faster switching times at the sameoperating voltages. The low glass transition temperatures (T_(g)'s) ofwater soluble binders, especially proteins, allow the polymeric bindermaterial to rearrange and thus shrink while water is evaporating fromthe binder/microcapsule layer during the drying step. The enhancedshrinkage of the polymeric binder thus produced results in the finalthickness of the binder/microcapsule layer being a smaller proportion ofthe initial thickness of the layer of microcapsules and binder solutionthan when a polymer latex is used as the binder solution, since when apolymer latex is dried, the water between the polymer particles isdriven off and the particles coalesce, but the polymer particlesthemselves, which typically have a T_(g) greater than those of watersoluble binders, are unable to shrink appreciably as the layer dries.

This difference between the behavior of electrophoretic media made withwater-dispersed and water soluble binders can be observedmicroscopically. FIG. 1 of the accompanying drawings is aphotomicrograph of a layer of microcapsules (average diameter about 35μm) and binder solution as initially deposited; FIG. 1 shows a layerused to form a water soluble binder electrophoretic medium of thepresent invention, but, at this stage of the process, there is littledifference between media formed with the two types of binders. As thelayer is dried and water is evaporated therefrom, a large amount ofshrinkage of the microcapsule/binder mixture occurs, resulting in a highdegree of deformation of the capsules, which are compressed into anessentially close packed dried film, of decreased film thickness, asshown in FIG. 2. (For discussion of the role of shrinkage of themicrocapsule/binder solution mixture, and consequent changes in theshape of the microcapsules, see the aforementioned U.S. Pat. Nos.6,067,185 and 6,392,785.) In contrast, as shown in FIG. 3, a similarmedium prepared with a water dispersed latex shows substantially lessdeformation of the capsules, and a less close packed structure. Theclose packed structure produced in the medium of the present inventionis advantageous because this structure improves the “active fraction” ofthe display, i.e., the proportion of the physical area of the displaywhich is covered by the microcapsules, and which thus changes opticalstate when an electric field is applied across the medium. It isdesirable that this active fraction be as large as possible, since thegaps between the microcapsules visible in FIG. 3, which do not changeoptical state when the medium is switched, reduce the contrast ratio ofa display incorporating the electrophoretic medium.

The following Example is given by way of illustration only to show apreferred water soluble binder electrophoretic medium.

EXAMPLE 4

A slurry of microcapsules were prepared substantially as described inthe aforementioned application Ser. No. 10/063,236, Paragraphs[0061]-[0068]. An aliquot (25 g) of the slurry, containing 80 percent byweight of microcapsules, was mixed with an aqueous gelatin solution (9 gof a 15 weight percent solution, equivalent to 1.33 g of gelatin) sothat the weight ratio of capsules to gelatin in the mixture was 15:1.Mixing was effected at 45° C. and the resultant mixture was maintainedat this temperature and stirred until completely homogeneous. Themixture was then coated on a polyester film using a draw bar at 2, 3 and4 mil (51, 76 and 101 μm respectively) settings. FIG. 1 illustrates theappearance of the wet 2 mil film at this point. The coated films wereallowed to dry in air for 10 minutes and then in an oven at 50° C. for15 minutes, after which the film produced at the 2 mil setting produceda dried film 25 μm thick, substantially the thickness desired; the driedfilm is shown in FIG. 2. To provide a control Example, the experimentwas repeated replacing the gelatin solution with an aqueous polyurethanelatex (NeoRez R 9320, available form NeoResins, Inc., Wilmington Mass.).FIG. 3 shows the dried polyurethane film corresponding to FIG. 2, and aspreviously mentioned this film shows much less satisfactory packing andcoverage than that in FIG. 2.

Swellable Wall Capsules

As already mentioned, this aspect of the present invention provides acapsule comprising a capsule wall and an internal phase encapsulated bythe capsule wall, the internal phase comprising a suspending fluid and aplurality of electrically charged particles; the capsule wall is formedof a material which is swellable by the suspending fluid.

Hitherto, capsules used for electrophoretic displays have had capsulewalls of a material that is not soluble in, and non-swellable by, thesuspending fluid within the capsule. The capsule walls thus spatiallycontain (encapsulate) the suspending fluid. A non-swellable capsule wallmaterial usually results in a mismatch of electrical properties betweenthe wall and the suspending fluid, and this mismatch can give rise toself-erasing under some conditions.

In contrast, in the swellable wall capsule of the present invention, thecapsule wall is formed from a material which is swellable by thesuspending fluid. Only slight swelling of the capsule wall is needed toproduce the advantages of the present invention, and excessive swellingshould be avoided since it may weaken the mechanical strength of thewall and consequently limit the mechanical integrity of theelectrophoretic medium. Using a swellable wall material in accordancewith this invention causes the capsule walls to become slightly swollenby the suspending fluid, thus making the volume resistivity of thecapsule walls similar to that of the volume resistivity of thesuspending fluid, and reducing the mismatch in electrical propertieswhich appears to be the cause of self-erasing.

Typically, the suspending fluid used in an electrophoretic medium is ahydrocarbon, typically an aliphatic hydrocarbon, alone or in admixturewith a halocarbon. Wall materials which can be slightly swelled by sucha hydrocarbon include silicones (for example poly(dimethyl siloxane) andpoly(trifluorobutyl methyl siloxane)), polymers derived from vinylicmonomers (for example poly(vinyl chloride), poly(butadiene),polyacrylates and polymethacrylates) and polyurethanes.

Although the wall material used is swellable by the suspending fluid,the binder which typically surrounds the capsules and the laminationadhesive typically provided adjacent this binder should not be soswellable. It is desirable that the suspending fluid be essentiallyconfined to the spaces occupied by the capsules and not migrate to otherparts of the electrophoretic medium or display, since migrating internalfluid may cause serious problems, such as partial or completedelamination of various layers of the display, unwanted color changes orchemical or electrochemical reactions (for example at the electrodes)which may adversely affect the operating performance and/or lifetime ofthe display. Using a binder and lamination adhesive which are notswellable by the suspending fluid ensures that, even though somesuspending fluid migrates into the capsule wall as that wall swells,this suspending fluid cannot migrate further to cause the aforementionedproblems.

The swellable wall capsules may be prepared by any of the variousmethods known in the art. For example, the capsules may be prepared bycoacervation of the wall material around preformed droplets of theinternal phase, or by the various extrusion techniques described in theaforementioned U.S. Pat. No. 6,377,387. It should be noted that,although the swellable wall material will have substantial affinity forthe suspending fluid (otherwise it would not be swellable by the fluid),the presence of significant amounts of wall material, or of componentsused to produce the wall material, in the suspending fluid within thefinal capsule should be avoided, since such material may adverselyaffect the electro-optic performance of the electrophoretic medium. Theprocess used to produce the capsules should be chosen with this in mind.More specifically, any monomers, oligomers or polymers which areproduced during formation of the capsule walls should be essentiallyinsoluble in the suspending fluid, or, if they are soluble, anencapsulation method should be chosen which allows for rapid reaction ofthe material to form the capsule wall once the material comes intocontact with the suspending fluid, thus ensuring that no more than aminimal amount of the material remains in the suspending fluid.Appropriate methods include coextrusion of the internal phase with asolution of the wall material under conditions which ensure rapidevaporation of the solvent from the wall material solution, orradiation-initiated polymerization of the wall material, preferably withultra-violet radiation.

The presently preferred method for preparing capsules with swellablewalls is concentric nozzle coextrusion, as described in detail in theaforementioned U.S. Pat. No. 6,377,387, with extrusion of the internalphase through the inner nozzle, and extrusion of a fluorosilicone (forexample, Dow Fluorgel 3-6679, available from Dow Chemical Company,Wilmington Del.) through the outer nozzle, followed by curing of theresultant capsule walls at elevated temperature. Alternatively, thesilicone could be replaced by an ultra-violet curable epoxy resin(several suitable resins are available from DSM desothech) followed byultra-violet irradiation of the coextruded internal phase/epoxydroplets.

The internal surface of the capsule wall may be provided with polymerchains which are solvated by the suspending fluid. The aforementioned2002/0185378 describes the advantages of providing polymer chains on thesurfaces of the electrophoretic particles themselves, and notes thatsuch polymer chains are desirably chosen so that they are highlysolvated by the suspending fluid so that they spread into a so-called“brush structure” which sterically stabilizes the particle in suspensionin the suspending fluid. The presence of such polymer chains on thesurfaces of the electrophoretic particles also helps to avoid theparticles sticking to the capsule wall, since the solvated polymer layerreduces the attractive force between the particle and the wall. However,additional protection against particles sticking to the capsule can beprovided by providing solvated polymer chains on the wall also, thusproviding two sets of polymer chains between the particle and the walland further reducing the attractive force therebetween.

Since, as already mentioned, the suspending fluid used in anencapsulated electrophoretic medium is typically an aliphatichydrocarbon, the polymer chains provided on the capsule wall shouldnormally be chosen to be highly solvated by such a hydrocarbon.Extensive guidance regarding preferred types of polymer chains is givenin the aforementioned 2002/0185378, and the same considerations apply topolymer chains provided on the capsule wall. In general, it is preferredthat the polymer chains have a main chain and a plurality of side chainsextending from the main chain, each of the side chains comprising atleast about four carbon atoms, thus providing a highly branchedstructure. Such a chain can be produced by polymerization of a monomerhaving a polymerizable group and a long alkyl chain, for example laurylmethacrylate.

Those skilled in the art of polymer synthesis will be familiar withnumerous methods which could be used to provide the polymer chains onthe capsule walls. In general, it is preferred to provide the polymerchains before the material is formed into the capsule wall, but we donot exclude the possibility of forming such polymer chains by reactionbetween a reactive site provided on the capsule wall and a reagentprovided in the suspending fluid itself.

As already indicated, the swellable wall capsule aspect of the presentinvention provides capsules which are less susceptible to self-erasingthan prior art capsules. In addition, the swellable wall capsule aspectof the present invention provides capsules in which the particles showreduced tendency to stick to the capsule wall.

Mixed Binder/Adhesive Electrophoretic Medium

As already indicated, the mixed binder/adhesive aspect of the presentinvention provides an electrophoretic medium in which at least one ofthe polymeric binder and (optional) lamination adhesive is formed from ablend of two (or more) materials, preferably polymers, the blend havinglower changes in volume resistivity with humidity than either componentseparately.

The preferred materials for use in such a mixed binder/adhesiveelectrophoretic medium are polyurethanes, especially polyurethanesproduced from aliphatic polyesters. Such polyurethanes are availablecommercially in the form of anionic aqueous dispersions, for example asNeoRez R 9621, R 9314 and R 9630, all from NeoResins, Inc.

It may at first glance appear surprising that a mixture of two or morematerials which themselves have volume resistivities which varysubstantially with relative humidity of the environment, can have avolume resistivity which varies much less with relative humidity.However, this apparent anomaly is explicable in terms of the chemistryof polyurethanes and other polymers used as binders and laminationadhesives (although the present invention is in no way limited by thefollowing explanation of the anomaly). Polyurethanes and otherwater-borne polymers of contain certain chemical segments, such ascarboxylic acid groups, and urethane and urea groupings, which aresusceptible to moisture uptake. When two or more of such materials aremixed, some of these chemical segments may react with each other andcross-link the materials; cross-linking is a common method of improvingthe resistance of a single polymer to moisture.

The following Examples are now given, though by way of illustrationonly, to show preferred blends which may be useful in mixedbinder/lamination adhesive electrophoretic media of the presentinvention.

EXAMPLE 5

This Example illustrates one specific blend of polyurethanes the volumeresistivity of which changes much less with relative humidity than doesthat of either component separately.

The polyurethanes used in this Example were NeoRez R 9314 and NeoRez9621, and a 3:1 w/w blend of the two polyurethanes. Films of all threematerials were coated on to an indium-tin oxide (ITO) coated polyesterfilm, dried, and a second ITO-coated polyester film laminated to thefirst so as to sandwich the polyurethane layer between the twoITO-layers. The resultant samples were placed in controlled humidityenvironments of from 20 to 90 percent relative humidity (RH) at roomtemperature (approximately 20° C.) and their volume resistivitiesmeasured at intervals, by impedance spectroscopy and current transientmeasurements, until they became stable, thus showing that the sampleswere in equilibrium with the controlled-humidity air surrounding them.All resistivity ratios reported are based on the resultant equilibriumvalues.

The results obtained are shown in FIG. 4 of the accompanying drawings,where the results are plotted as the volume resistivity at 20 percent RHdivided by that at 90 percent RH. From this Figure, it will be seen thatthe blend of R 9314 and R 9621 had a volume resistivity which was muchless sensitive to humidity that that of either component alone, thevolume resistivity of the blend varying by only a factor of about 3between 20 and 90 percent RH, whereas the components varied by factorsof about 10 (R 9314) and 50 (R 9621) respectively.

EXAMPLE 6

This Example illustrates a second specific blend of polyurethanes thevolume resistivity of which changes much less with relative humiditythan does that of either component separately.

Example 5 was repeated except that the materials tested were NeoRez R9314, NeoRez R 9630, and a 3:2 w/w blend of the two polyurethanes. Theresults obtained are shown in FIG. 5 of the accompanying drawings. Fromthis Figure, it will be seen that the blend of R 9314 and R 9630 had avolume resistivity which was much less sensitive to humidity that thatof either component alone, the volume resistivity of the blend varyingby only a factor of about 4.5 between 20 and 90 percent RH, whereas thecomponents varied by factors of about 13 (R 9314) and 30 (R 9630)respectively.

From the foregoing, it will be seen that the mixed binder/adhesiveaspect of the present invention provides electrophoretic media which aremuch less sensitive to relative humidity than prior art media relyingupon simple polyurethane binders and lamination adhesives. Thus, theelectrophoretic media of the present invention can operate over anincreased range of relative humidity.

Apart from the modification of the capsules walls in accordance with thepresent invention, the mixed binder/adhesive electrophoretic media anddisplays of this invention can make use of any of the materials andproduction techniques described in the aforementioned MIT and E Inkapplications, to which the reader is referred for additional details.

Two Prepolymer Process

As already indicated, capsules produced by the complex coacervation ofgelatin and acacia, as described in some of the aforementioned E Ink andMIT patents and published applications, have been found to be sensitiveto the relative humidity of the environment. Consequently, displaysbased upon such capsules may operate only within a limited relativehumidity range, or relatively complex sealing and/or barrierarrangements may be needed to enable operation over a wider relativehumidity range. The two prepolymer process of the present inventionseeks to provide capsules which are less humidity sensitive thancapsules produced by the complex coacervation of gelatin and acacia.

In general terms, the two prepolymer process involves dissolving ordispersing a first prepolymer having first reactive groups in a fluid,for example, the suspending fluid to be used in an electrophoreticmedium; obviously, this fluid may contain additional components, forexample one or more types of electrically charged or chargeableparticles which will eventually serve as the electrophoretic particlesof the medium. Separately, a second prepolymer having second reactivegroups is dissolved or dispersed in a dispersion medium, which issubstantially immiscible with the fluid containing the first prepolymer,and is typically aqueous. The prepolymers are chosen so that the firstand second reactive groups will react together so as to join theprepolymers into a polymer which forms a suitable capsule wall material.The fluid containing the first prepolymer is dispersed as a plurality ofdroplets in the dispersion medium, so that at the interface between thetwo phases the reaction between the two prepolymers forms a polymer,typically formed by cross-linking of the two prepolymers, and thispolymer forms a capsule wall within which the fluid is encapsulated.

It will be appreciated that the prepolymers should be chosen so thateither there is no substantial amount of first prepolymer left withinthe capsules after the encapsulation is complete, or the amount of firstprepolymer so left does not adversely affect the expected properties ofthe capsule. For example, when the two prepolymer process is used toform an encapsulated electrophoretic medium, any first prepolymer leftwithin the final capsules should not interfere with the electricalcharging of the electrophoretic particles essential for the properoperation of the medium.

In one experimental demonstration, the two prepolymer process was usedto prepare electrophoretic capsules having an internal phase comprisingtitania and carbon black particles dispersed in a hydrocarbon suspendingfluid. The prepolymers used were two commercial prepolymers, namelypoly(isoprene-graft-maleic anhydride) (PI-g-MA) (hydrocarbon soluble)and polyethyleneimine (PEI) (water soluble). Polymer formation betweenthese two prepolymer occurs by reaction of the amine groups on the PEIwith the anhydride groupings on the PI-g-MA to form amide linkages,which cause the polymer not to be soluble in either hydrocarbon orwater. A typical encapsulation procedure is as follows.

An electrophoretic medium internal phase (hydrocarbon containing titaniaand carbon black particles) was emulsified in water in the presence ofnon-ionic surfactant for one hour with mechanical agitation to form ahydrocarbon-in-water emulsion. To this emulsion, there was addeddropwise an aqueous solution of PEI, with continued mechanicalagitation. The reaction was allowed to proceed for 15 minutes after theaddition of the PEI had been completed and the resultant capsules wereseparated from the liquid by centrifugation. A portion of the capsuleswere coated on to a glass slide and dried.

FIG. 6 of the accompanying drawings is an optical photomicrograph of thecapsules formed in reaction is suspension in the liquid. It will be seenfrom this Figure that the particles do not coalesce even in closecontact, a strong indication that a substantial capsule wall has beenformed surrounding the internal phase. FIG. 7 is a similar opticalphotomicrograph of the dried film and illustrates the close packing ofthe capsules, suggesting they are highly deformable.

The two prepolymer process of the present invention has the advantagesthat it can be carried out at ambient temperature, thus avoiding theheating and cooling of the reaction mixture required in otherencapsulation processes, the consequent reduction in processing time.Furthermore, the properties of the capsule wall formed by easily beadjusted by varying the prepolymers and the reaction conditions. Theprocess can also produce capsules with substantially reduced humiditysensitivity.

General Considerations Concerning Capsules

It appears (although this invention is in no way limited by any theoryas to such matters) that this service life of electrophoretic displaysis limited by factors such as sticking of the electrophoretic particlesto the capsule wall, and the tendency of particles to aggregate intoclusters which prevent the particles completing the movements necessaryfor switching of the display between its optical states. In this regard,opposite charge dual particle electrophoretic displays pose aparticularly difficult problem, since inherently oppositely chargedparticles in close proximity to one another will be electrostaticallyattracted to each other and will display a strong tendency to formstable aggregates.

Substantial improvements in the overall performance of electrophoreticdisplays may be expected from alternative encapsulation materials andprocesses. As previously described, in the most well developedencapsulated electrophoretic displays, capsules may be constructedcontaining a pigmented dielectric fluid as an internal phase surroundedby a thin shell of a tough, impermeable polymeric material. Thedimensions of the encapsulated particles are typically on the order of50 to a few hundred microns in size; the shell thickness is 10 nm orless. For application in electrophoretic displays, the capsule mustsatisfy a long list of requirements to guarantee satisfactoryperformance. Among these requirements are the following:

1. The encapsulation procedure must be reproducible and manufacturable,involve inexpensive raw materials, and yield capsules that are totallyimpermeable to the contents.

2. The capsules must be amenable to coating. While the properties of theencapsulated dispersion that allow facile, uniform coating are notentirely understood, one property that is important is flexibility ofthe capsule wall. If the wall is too rigid, the coating suspension showssevere shear-thickening rheological behavior, and is either impossibleto coat because of hopper jamming or yields very non-uniform coatings.Flexibility of the capsule wall also allows closer packing in thecoating, and thus yields displays with improved optical properties.

3. The capsule wall must have mechanical, optical, and electricalproperties that allow the construction of a durable display with rapidresponse at low driving voltages. In particular, the shell must betolerant to mechanical deformation (this is especially important forflexible display applications) and must not be appreciably colored oropaque. Also, the electrical resistance of the shell wall material mustbe high; a capsule wall with poor electrical properties can short outthe display.

4. The capsule wall must maintain its properties over a wide range ofoperating conditions. As already discussed, the response of the capsuleto changes in humidity is especially problematical, since it has beenfound to be difficult to achieve simultaneously all of thecharacteristics listed above with a capsule wall composition whoseelectrical conductivity is sufficiently insensitive to high ambienthumidity. Improvements in the environmental sensitivity of the capsulewall represent a major contribution to the robustness of the display.

For the foregoing reasons, encapsulation technology is seen as thelargest single barrier to achieving a rugged, truly environmentallyinsensitive electrophoretic display.

The following approaches may be used to improve the durability androbustness of electrophoretic displays:

1. Synthetic manipulation of the coacervate polymers to reduce theirhumidity sensitivity, particularly by the incorporation of hydrophobicmoieties into one or both of them; alternatively, complete replacementof one or more of the natural polymers with specially designed syntheticpolymers, with carefully balanced charge and hydrophobic character;

2. Other encapsulation methods, specifically modifications thereof thatprovide improved performance in electrophoretic display function andmanufacture; and

3. Replacement of organic capsule wall material with inorganic materialssuch as silica or organic/inorganic composites or hybrid materials,using technology for surface modification (see the aforementioned2002/0185378), or modifications of other procedures disclosed in theliterature.

Polymeric Materials for Coacervate Encapsulation

Gelatin can form a complex coacervate phase with a wide variety ofpolyanions. The structure of the polyanion can have a substantial effecton the electrical properties of the wall material, and on theenvironmental sensitivity of the resulting display. This approachprovides a simple solution to the environmental sensitivity issue andimposes the least disruption on existing display manufacturingprocesses.

Chemical intuition suggests that replacement of gum acacia with a lesshydrophilic synthetic polymer would reduce the affinity of thecoacervate shell to water. To this end, this invention relates to theuse of polyanions with moderately hydrophobic backbone substitution. Asalready discussed above with regard to the polyanionic-basedencapsulation materials of the present invention, one particularlyattractive class of polyanionic materials are the polycarboxylic acidpolymers prepared from alt-poly(olefin-maleic anhydride). Thesematerials are commercially available with a range of hydrophobic sidechains, depending on the choice of olefin co-monomer. Another class ofreadily available materials suitable for the intended applicationcomprise co-(meth)acrylate ester-(meth)acrylic acid copolymers.Sulfonate polymers based on copolymers of styrene or acrylate esterswith styrene sulfonic acid are also attractive alternatives.

This invention extends to modification of the anionic component of thecoacervate, and modification of gelatin. The simplest modificationinvolves acylation of lysine ammonium side groups to yield (neutral)amide functions. Since this reaction destroys the cationic groups thatallow coacervation with polyanions to occur, only partial modificationby this route is possible. Alternatively, esterification or amidationreactions with carboxyl function on the gelatin are possible.

Other Encapsulation Techniques

Complex coacervation techniques as described above yield capsule wallsthat inevitably comprise, at least in part, polar functionality, i.e.,the ion pairs that cause complex formation. The ability to control theelectrical resistance of capsule walls formed by this process istherefore inherently limited. Encapsulation materials formed by in situor interfacial polymerization processes do not necessarily involvehighly polar components, and, further can be made with high cross-linkdensities by incorporation of multifunctional monomers. This inventiontherefore extends to the use of several of these processes for theconstruction of capsules for electrophoretic display applications.

Two limitations on the utility of these capsules are apparent. First, ifthe capsule wall is too hydrophobic, it become permeable to theencapsulated dielectric fluid in the internal phase. This problem can inpart be overcome by higher cross-linking densities, and by the use ofwall-forming monomers with limited solubility for hydrocarbons. However,high cross-linking density tends to yield rigid, non-deformablecapsules, and as we have previously mentioned, these materials are notonly difficult to coat, but also yield displays with poor capsulepacking and degraded electro-optical properties. An optimized capsulereflect a proper balance of polarity of the wall materials and theirmechanical properties.

Reliable methods for encapsulation by in situ and interfacialpolymerization are well-known in the literature; see, for example, theaforementioned U.S. Pat. No. 6,377,387. Among the attractivealternatives are formation of capsules by amide-formaldehydecopolymerization and the use of amine plus acid chloride and amine plusisocyanate reactions. The first of these methods (urea-formaldehyde ormelamine-formaldehyde) has been shown to yield highly rigid capsulesthat are not easily compatible with the remainder of a typicalencapsulated electrophoretic display manufacturing process. However,this process can be modified to provide less rigid shells by usingblocked urea derivatives (e.g., N,N″-dimethylurea) or simple primary orsecondary amides as co-reactants to reduce the degree of cross-linking.Because of the toxicity of formaldehyde, blocked formaldehyde precursors(hexamethylene tetramine, formaldehyde sulfonate adduct, etc.) may beused as synthetic alternatives. It should be noted thaturea-formaldehyde polymerization has also been shown to reinforcegelatin/acacia and other coacervate capsules, providing an improvedencapsulation procedure. Such a hybrid capsule wall should show improvedRH sensitivity in a electrophoretic display device. Other more exotichybrid structures are considered below.

Because the binders and adhesives presently used in encapsulatedelectrophoretic displays are typically polyurethane-based, encapsulationusing polyisocyanate precursors has advantages with regard to melt andcoating compatibility. The procedure of Schur, et al. (U.S. Pat. No.4,285,720), yielding a poly(substituted urea) shell by self-hydrolysisof the isocyanate is attractive because of its simplicity. Theproperties of the shell can in principle be modified by incorporation ofdi- or polyfunctional amines in the aqueous phase during encapsulation.By appropriate choice of the structure of the amine(s) and itsconcentration, a wide variety of shell properties are obtainable.

A third chemistry for interfacial polymerization includes thecombination of hydrophobic poly(acyl halide) derivatives in the oilphase combined with polyamino compounds in the aqueous phase to yieldpolyamide shells. This chemistry, also well documented in the literatureas an encapsulation technique, further expands the range of chemicalconstitutions available for microencapsulated electrophoretic displaydevices. Which chemistry is most advantageous is any specificapplication is determined by a combination of studies of syntheticfacility and device function, particularly in the context ofenvironmental sensitivity.

Organic/Inorganic Hybrid Capsules

The ultimate in shell impermeability and low resistivity is obtainedusing a capsule walls based on inorganic polymeric materials. As anexample, a condensed silica shell can be made completely impermeable toboth water and hydrophobic materials, and also have very lowconductivity. Such a shell would also, however, be expected to be veryrigid, with attendant manufacturing difficulties. Encapsulation usinginorganic/organic hybrid structures allows the permeability andelectrical properties to improve relative to the organic materials,while a degree of flexibility is provided by the organic component. Anumber of routes to such hybrid structures can be used. Surface-modifiedsilica particles with incorporated amine functionality may beincorporated with either polyisocyanate or poly(acyl halide) interfacialpolymerization procedure as described above. Alternatively,polycarboxylate polymers bind strongly to many inorganic colloidalmaterials (e.g., alumina). Appropriate choice of polycarboxylateconcentration and structure can lead to a surface-active aggregatestructure that will adsorb strongly at the oil-water interface (suchaggregates are known to be involved in the formation of so-calledPickering emulsions, oil-in-water emulsions stabilized by inorganiccolloids). Once formed, the interfacial structure can be reinforced andthickened by successive deposition of further layers of alumina andpolymer, either sequentially or in a single step. Most simply, colloidalmaterials with anionic surfaces, notably silica at pH values nearneutrality, can be regarded as polyanionic materials that are capable offorming coacervate phases with gelatin or other polymers with cationicsubstituent groups just as gum acacia or polycarboxylates do. Theobservation by Wang and Harrison (G. Wang and A. Harrison, J. ColloidInterface Sci., 1999, 217, 203) that an adsorbed gelatin layer acts as apriming agent for the encapsulation of iron particles by silica usingthe Stöber process supports this view.

Such novel hybrid coacervate encapsulation media show interestingproperties in microencapsulated electrophoretic display devices. Atleast part of the RH sensitivity of conventional coacervate microcapsulewalls is the result of the swellability of the anionic component. To theextent that the water affinity of this component is reduced oreliminated, water uptake at high RH, with its attendant reduction inresistance and increased permeability, is reduced.

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention. Accordingly, the foregoing description is to be construed inan illustrative and not in a limitative sense.

1. An electrophoretic medium comprising a plurality of capsules in apolymeric binder, each capsule comprising a capsule wall and an internalphase encapsulated by the capsule wall, the internal phase comprising asuspending fluid and a plurality of electrically charged particlessuspended in the suspending fluid and capable of moving therethroughupon application of an electric field to the capsule, the polymericbinder being a water soluble polymer.
 2. An electrophoretic mediumaccording to claim 1 in which the water soluble polymer comprisesgelatin.
 3. An electrophoretic medium according to claim 1 in which thecapsule wall comprises a coacervate of gelatin and acacia.
 4. A capsulecomprising a capsule wall and an internal phase encapsulated by thecapsule wall, the internal phase comprising a suspending fluid and aplurality of electrically charged particles suspended in the suspendingfluid and capable of moving therethrough upon application of an electricfield to the capsule, the capsule wall being formed of a material whichis swellable by the suspending fluid.
 5. A capsule according to claim 4in which the internal phase comprises a hydrocarbon and the capsule wallmaterial comprises any one or more of a silicone, a polymer derived froma vinylic monomer, and a polyurethane.
 6. A capsule according to claim 4in which the internal surface of the capsule wall bears at least onepolymer chain which is solvated by the suspending fluid.
 7. A capsuleaccording to claim 6 in which the internal phase comprises a hydrocarbonand the polymer chain has a main chain and a plurality of side chainsextending from the main chain, each of the side chains comprising atleast about four carbon atoms.
 8. An electrophoretic medium comprising aplurality of capsules according to claim 4 and a binder surrounding thecapsules.
 9. An electrophoretic medium according to claim 8 in which thebinder is substantially not swellable by the suspending fluid.
 10. Anelectrophoretic medium according to claim 9 further comprising a layerof an adhesive, the adhesive not being substantially swellable by thesuspending fluid.
 11. An electrophoretic medium comprising a pluralityof capsules in a polymeric binder, each capsule comprising a capsulewall and an internal phase encapsulated by the capsule wall, theinternal phase comprising a suspending fluid and a plurality ofelectrically charged particles suspended in the suspending fluid andcapable of moving therethrough upon application of an electric field tomicrocapsule, the electrophoretic medium also optionally comprising alayer of a lamination adhesive in contact with the binder, at least oneof the polymeric binder and lamination adhesive being formed from ablend of a plurality of materials, the blend having lower changes involume resistivity with humidity than any of its component materialsseparately.
 12. An electrophoretic medium according to claim 111 inwhich the blend comprises a plurality of polyurethanes.